Zirconium is widely used today as cladding in nuclear fuel rods. However, the zirconium-containing ores that are the best source of this zirconium also contain hafnium. Separation of the zirconium from the hafnium must be accomplished so that the zirconium is in a form in which it can be used for nuclear fuel rods or for other purposes requiring substantially hafnium-free zirconium. Nuclear grade zirconium standards typically require no more than 30 ppm hafnium.
Several processes for the production of hafnium-free zirconium are known. One known process employs a zirconium/hafnium separation process, which uses tributyl phosphate (TBP) solvent and a zirconyl nitrate feed. Another known zirconium/hafnium separation process is a distillation based separation process. A third zirconium/hafnium separation process, which is currently used to produce most of the nuclear grade zirconium available for use as fuel rod cladding and the like, was developed by the United States Bureau of Mines and is described in the publication entitled "Zirconium-Hafnium Separation," by W. A. Stickney, Bureau of Mines Report of Investigations 5499 (1949). In this process, zirconium and hafnium are separated from a chloride media using a thiocyanate complexing agent and a methylisobutyl ketone (mibK) solvent.
The U.S. Bureau of Mines process requires the preparation of a zirconium feed by dissolving ZrCl.sub.4 in water. This feed is highly acid because of the presence of HCl produced by hydrolysis, and the zirconium concentration is low because of the high acidity of the system. The zirconium feed is filtered, and iron, which must be removed, is removed by contact with mibK in a packed column to produce an iron free feed. The iron loaded mibK is washed with water, and the solvent regenerated. The iron free feed is mixed with ammonium thiocyanate (NH.sub.4 SCN) and aqueous solution recycled from a stripper. The washed mibK, the stripper aqueous and the iron-free Zr feed are combined to form the extractor aqueous. Solvent, which is a 50/50 mixture of solvent from the thiocyanate recovery system, and scrubbed mibK is fed to an extractor. The aqueous stream exiting the extractor is then fed to a zirconium raffinate thiocyanate recovery section. Thiocyanate is extracted as HSCN by the addition of HCl, and the aqueous zirconium raffinate is steam stripped to recover the 2% dissolved mibK. Finally, zirconium is precipitated as a 5/2 sulfate for additional purification and calcining.
In the foregoing process, the aqueous zirconium feed is typically run through a liquid-liquid type extraction circuit including a series of packed columns where the aqueous zirconium-containing liquid flows countercurrent to an organic liquid, usually mibK solvent, which typically flows continuously. However, these packed columns suffer from some significant disadvantages. They are difficult to control, and there is little or no relationship between the flow rate and column transfer capability.
While the aforementioned process is an effective method for producing substantially hafnium-free zirconium, it is somewhat inefficient. The solvent leaving the extractor can contain up to 50% of the zirconium in the feed. This zirconium must be stripped with HCl and returned as the stripper recycle. Scrubbing of the stripped solvent with sulfuric acid to remove hafnium is then required. The hafnium raffinate produced is first contacted with mibK to recover the thiocyanate present and then steam stripped to remove mibK before precipitation with ammonia. Additionally, NH.sub.4 Cl, which is generated as a waste product, must be disposed of.
U.S. Pat. Nos. 3,006,719 and 3,069,232 are directed to improvements of the Bureau of Mines zirconium extraction process. U.S. Pat. No. 3,006,719 is directed specifically to the removal of iron impurities from the Zr/Hf feed, and U.S. Pat. No. 3,069,232 is directed to the recovery of hafnium values. Neither of the processes disclosed in these patents, however, improves the efficiency of the zirconium/hafnium separation process to produce nuclear grade zirconium at a lower cost and with smaller in-process inventories than the Bureau of Mines process.
The available, commercial zirconium separation processes employing thiocyanate regeneration systems have not appreciated the importance of system design on the purity of the mibK solvent and the regenerated NH.sub.4 SCN. As a result, these processes have produced mibK with unacceptably high thiocyanate levels. A concentration significantly below the 0.1M SCN produced by these systems is desirable to insure the production of high quality nuclear grade zirconium.
The prior art, therefore, fails to disclose an efficient extraction process for producing high quality, substantially hafnium-free nuclear grade zirconium. The available prior art processes, moreover, are inefficient and slow, require substantial in-process inventories of mibK and thiocyanate and, as a result, are costly.